1. Technical Field of the Invention
The present invention generally relates to the wireless telecommunications field and, in particular, to a communications terminal having a receiver and method for substantially cancelling known interferers from a digitally translated intermediate frequency (IF) signal.
2. Description of Related Art
A mobile phone incorporates many components including a receiver that operates to demodulate signals received from a transmitter by removing a carrier signal and outputting a desired signal. Receivers commonly used today include homodyne receivers, heterodyne receivers, superheterodyne receivers and double superheterodyne receivers.
Of course, the above-mentioned receivers are sensitive to interference from external jamming equipment and internal equipment. For example, the mobile phone used in accordance with the Global System for Mobile Communications (GSM) Standard has an interference problem attributable to a 13 MHZ internal clock which generates 72nd and 73rd harmonics that interfere with a desired signal on channels 5 and 70, respectively.
Currently, the interference caused by the clock harmonics may be addressed by paying particular attention to the shielding and decoupling within the mobile phone which can be a very expensive and complicated task. Another technique used today to cancel the known interferers (e.g., 72nd and 73rd harmonics) caused by the 13 MHZ internal clock, is to in the receiver translate in the analog domain the signals received from the antenna down to baseband so that the clock harmonics are translated to a direct current (DC) offset voltage which is then removed by analog or digital DC-offset cancellation techniques. Unfortunately, the receivers that translate the received signals down to baseband in the analog domain must also remove a lot of extraneous DC offset voltages that are created by mixers used to produce the analog baseband signals or by analog circuitry used to process the analog baseband signals. Extraneous DC offset voltages may also be due to component mismatches and carrier leakage. Consequently, the extraneous DC offset voltages must be removed since they can be larger than the desired signal. The extraneous DC offsets is one reason why digitizing an IF signal is more attractive than digitizing baseband signals.
Accordingly, there is a need for a communications terminal having a receiver and method that effectively removes known in-band interferers from received IF signals that are initially separated from baseband signals in analog domain and later digitized in the digital domain, so as to avoid the undesirable extraneous DC offset voltages associated with traditional communications terminals. This need and other needs are satisfied by the communications terminal and method of the present invention.
The present invention is a communications terminal having a receiver and method that substantially removes a known interferer from a received IF signal that is digitized. More specifically, the receiver includes an antenna for receiving a signal, and at least one combination of a mixer and filter for translating the signal in the analog domain to an IF signal while maintaining separation from the baseband. The receiver also includes a digitizer for digitally translating the IF signal containing the known interferer from the analog domain into the digital domain, and an interference cancellation system for removing the known interferer from the digitally translated IF signal by utilizing some signal processing means.
In accordance with the present invention, there is provided a method and receiver using a digital DC offset compensator to remove from a digitized IF signal a known interferer including a centered unmodulated interferer caused by the 72nd and 73rd harmonics of a 13 MHZ internal clock.
Also in accordance with the present invention, there is provided a method and receiver using a digital signal processing block to remove from a digitized IF signal a known interferer including an off-centered unmodulated interferer or a modulated interferer each having a known waveform.